The present disclosure relates to a differential transformer type magnetic sensor using planar coils.
In an image forming apparatus using a toner as a developer, a magnetic sensor is used to detect the remaining amount and density of the toner. There are various types of magnetic sensors. A differential transformer type magnetic sensor is so configured that a drive coil, a differential coil which functions as a detection coil and a differential coil which functions as a reference coil are arranged on the same core.
A differential transformer type magnetic sensor in which a first coil (drive coil), a second coil (differential coil), a third coil (differential coil) and a fourth coil (drive coil) are respectively arranged on first, second, third and fourth layers and insulating substrates are arranged between the respective layers has been proposed as an example.
A differential transformer type magnetic sensor in which a first coil (drive coil) and a third coil (differential coil) are arranged side by side on one surface of an insulating magnetic substrate and a second coil (drive coil) and a fourth coil (differential coil) are arranged side by side on the other surface of the insulating magnetic substrate has been proposed as another example.
Since the first to fourth coils are arranged one over another via the substrates in the one example of the differential transformer type magnetic sensor, a distance between the first coil (drive coil) and the second coil (differential coil) and that between the third coil (differential coil) and the fourth coil (drive coil) can be approximated to each other. Thus, magnetic coupling between the first and second coils and that between the third and fourth coils can be increased. Therefore, a highly accurate magnetic sensor can be realized. However, four layers of substrates are necessary, which constitutes an obstacle to the miniaturization of the magnetic sensor and increases the production cost of the magnetic sensor.
Since two layers (both surfaces) of substrates are used in the other example of the differential transformer type magnetic sensor, the production cost of the magnetic sensor can be reduced. However, the first coil (drive coil) and the third coil (differential coil) are arranged side by side on the one surface of the substrate and the second coil (drive coil) and the fourth coil (differential coil) are arranged side by side on the other side of the substrate. The first and third coils are not facing each other, and the second and fourth coils are not facing each other. Thus, magnetic coupling between the first and third coils and that between the second and fourth coils cannot be increased. Therefore, a highly accurate magnetic sensor cannot be realized.
Further, a differential transformer type magnetic sensor necessitates a zero adjustment, i.e. an adjustment to balance between an electromotive force generated in one differential coil (detection coil) and that generated in the other differential coil (reference coil). The zero adjustment can be made by providing the magnetic sensor with a mechanism for adjusting the position of a core of a differential transformer and by adjusting the position of the core. However, in the case of a differential transformer using planar coils patterned on a printed circuit board as a drive coil and differential coils, it is difficult to provide the above mechanism on the printed circuit board.
An object of the present disclosure is to provide a highly accurate differential transformer type magnetic sensor while realizing a configuration using planar coils arranged on a substrate as a drive coil and differential coils.